We must now find the meaning of the point D. Suppose the pure [alpha]- or pure [beta]-form heated to the temperature _t'_, and the temperature maintained constant until the liquid has the composition _x'_ corresponding to the equilibrium at that temperature. If the temperature is now allowed to fall sufficiently slowly so that the condition of equilibrium is continually readjusted as the temperature changes, the composition of the solution will gradually alter as represented by the curve _x'_D. Since D is on the freezing point curve of pure [alpha], this form will be deposited on cooling; and since D is also on the equilibrium curve of the liquid, D is the only point at which solid can exist in stable equilibrium with the liquid phase. (The vapour phase may be omitted from consideration, as we shall suppose the experiments carried out in open vessels.) All systems consisting of the two hylotropic[281] isomeric substances [alpha] and [beta] will, therefore, ultimately freeze at the point D, which is called the "natural" freezing point[282] of the system; provided, of course, that sufficient time is allowed for equilibrium to be established. From this it is apparent that _the stable modification at temperatures in the neighbourhood of the melting point is that which is in equilibrium with the liquid phase at the natural freezing point_. From what has been said, it will be easy to predict what will be the behaviour of the system under different conditions. If pure [alpha] is heated, a temperature will be reached at which it will melt, but this melting point will be sharp only if the velocity of isomeric transformation is comparatively slow; _i.e._ slow in comparison with the determination of the melting point. If the substance be maintained in the fused condition for some time, a certain amount of the [beta] modification will be formed, and on lowering the temperature the pure [alpha] form will be deposited, not at the temperature of the melting point, but at some lower temperature depending on the concentration of the [beta] modification in the liquid phase. If isomeric transformation {199} takes place slowly in comparison with the rate at which deposition of the solid occurs, the liquid will become increasingly rich in the [beta] modification, and the freezing point will, therefore, sink continuously. At the eutectic point, however, the [beta] modification will also be deposited, and the temperature will remain constant until all has become solid. If, on the other hand, the velocity of transformation is sufficiently rapid, then as quickly as the [alpha] modification is deposited, the equilibrium between the two isomeric forms in the liquid phase will continuously readjust itself, and the end-point of solidification will be the natural freezing point.